Halliday & Matthiessen (2014: 31):
Why this rather unwieldy term ‘metafunction?’ We could have called them simply ‘functions’; however, there is a long tradition of talking about the functions of language in contexts where ‘function’ simply means purpose or way of using language, and has no significance for the analysis of language itself. But the systemic analysis shows that functionality is intrinsic to language: that is to say, the entire architecture of language is arranged along functional lines. Language is as it is because of the functions in which it has evolved in the human species. The term ‘metafunction’ was adopted to suggest that function was an integral component within the overall theory.